1. Field of the Invention
The invention relates to an apparatus for measuring the draw resistance of a rod of tobacco fibres, comprising a source for a gas stream passing through the rod and a measuring-value transducer for determining the pressure drop of the gas stream occuring with constant gas volume at the rod.
2. Description of the Prior Art
In the quality control of cigarettes the draw resistance of the cigarette is of great importance because from draw-resistance measurements, it is possible to determine in a relatively simple manner whether a cigarette permits the desired draw or puff volume. The draw volume of a cigarette is in turn governed on the one hand by the manner in which the smoker smokes the cigarette and on the other hand by its draw resistance.
Thus, to ensure for the smoker of a given brand cigarette always the same draw volume--keeping the subjective parameter, that is the manner how the smoker smokes the cigarette, constant--the draw resistance must be kept to a fixed value.
To ensure that this constant draw-resistance value is maintained, individual cigarettes are taken in random checks from the every day production and their draw resistance is determined with a device as described, for example, in the article "The Measurement of Smoking Parameters with the Aid of Parameter Converters", published in "Beitrage zur Tabakforschung", Volume 6, No. 1, July 1971. In this method the cigarette is connected by means of a rubber lip to a smoking machine which, for example, draws a gas flow with constant volume through the cigarette. In accordance with the usual standardization, the constant flow volume at the exit side should be 17.5 cm.sup.3 /sec.
Due to the flow resistance of the cigarette, a pressure drop occurs between the entry side and exit side of the gas flow and is referred to as "draw resistance"; it can be measured by a pressure transducer.
Alternatively, it is fundamentally also possible to measure the gas volume passing with constant pressure drop through the cigarette.
These apparatuses are however, only suitable for carrying out random checks, i.e. they cannot be used for every day, continuous measurement of the draw resistance of all cigarettes, as for example those made by a certain cigarette machine, because each cigarette must be inserted into the apparatus and measured individually. These random checks can only be carried out at predetermined intervals of time so that in general it is not possible to react immediately to a detected change of the draw resistance of the cigarette by appropriate regulation of the production.
British Pat. No. 1,588,506 discloses an apparatus for measuring the draw resistance of a rod of filter fibres, with which the pressure drop at the rod is determined in a region at which the rod has reached almost its final cross-section. The intention is to detect the pressure drop at a point behind the smallest cross-section of the rod of filter material because there the filter fibres have reached their final position and thus can no longer be displaced by the air passing through the rod. The main compression of the rod is by means of a wheel, whilst the shaping and subsequent compression is carried out by a format finger which is arranged in the movement direction of the rod behind the wheel.
An apparatus for measuring the draw resistance of a rod of tobacco fibres is disclosed in German Auslegeschrift No. 1 166 069 and comprises a source for a gas flow passing through the rod and a measuring-value transducer for determining the pressure drop of the gas flow with constant gas volume occuring at the rod or of the gas volume with constant pressure drop passing through the rod. A pump passes the air through the tobacco stream into an enclosed space of constant volume which is formed between a trough, through which the format belt draws the tobacco stream and a member opposite said trough, for instance a pressure shoe or a tongue known per se.
Admittedly, this permits the continuous measurement of the draw resistance of a rod of tobacco fibres., the value determined is also specific for the rod of tobacco fibres so that the determined draw resistance can be used for process control purposes. A disadvantage, however, is that the draw resistance of the rod of tobacco fibres thus determined does not correlate with the draw resistance of the finished cigarette, which is a very important product parameter because it is an essential characteristic of a specific cigarette type.